Visual search in the real world: evidence for the formation of distractor representations

Visual search in the real world often requires that we search the same environment a number of times for different targets. What is the fate of information about fixated distractor objects during these searches? Here, participants searched the same array of real objects on a tabletop twice for two different targets successively whilst wearing a head-mounted eye-tracker. We found that fixating an object when it was a distractor in the first search facilitated search for that same object when it became the target in the second search. The results suggest that the location and identity of fixated distractor objects are represented to a level that guides subsequent searches, even when this information is not required at the time of fixation.     

The influence of multiple readings on the missing-letter effect revisited

In searching for a target letter while reading, participants make more omissions when the target letter is embedded in frequent function words than when it is embedded in less frequent content words. According to the guidance-organization (GO) model, this occurs because high-frequency function words are processed faster than low-frequency content words, leaving less time available for letter processing. We tested this hypothesis in three experiments by increasing word-processing speed through text repetition, which should translate into higher omission rates. Participants either read the text and searched for the target letter once or read the text three times and searched for a target letter on all readings or the final reading only. In all the experiments in which participants could not anticipate the target letter to be used, results revealed the presence of a large missing-letter effect that was unaffected by familiarity with the text. In addition, when participants knew from the start the target letter to be used on the final reading, the missing-letter effect was eliminated. Repeated search of the same text for different targets increased omissions equally for function words and content words, but this finding was present even when a new text was used, suggesting that repetition of the search task, rather than familiarity with the text, was responsible.     

When does repeated search in scenes involve memory? Looking at versus looking for objects in scenes

One might assume that familiarity with a scene or previous encounters with objects embedded in a scene would benefit subsequent search for those items. However, in a series of experiments we show that this is not the case: When participants were asked to subsequently search for multiple objects in the same scene, search performance remained essentially unchanged over the course of searches despite increasing scene familiarity. Similarly, looking at target objects during previews, which included letter search, 30 seconds of free viewing, or even 30 seconds of memorizing a scene, also did not benefit search for the same objects later on. However, when the same object was searched for again memory for the previous search was capable of producing very substantial speeding of search despite many different intervening searches. This was especially the case when the previous search engagement had been active rather than supported by a cue. While these search benefits speak to the strength of memory-guided search when the same search target is repeated, the lack of memory guidance during initial object searches-despite previous encounters with the target objects-demonstrates the dominance of guidance by generic scene knowledge in real-world search.     

Effects of targets embedded within words in a visual search task

Visual search performance can be negatively affected when both targets and distracters share a dimension relevant to the task. This study examined if visual search performance would be influenced by distracters that affect a dimension irrelevant from the task. In Experiment 1 within the letter string of a letter search task, target letters were embedded within a word. Experiment 2 compared targets embedded in words to targets embedded in nonwords. Experiment 3 compared targets embedded in words to a condition in which a word was present in a letter string, but the target letter, although in the letter string, was not embedded within the word. The results showed that visual search performance was negatively affected when a target appeared within a high frequency word. These results suggest that the interaction and effectiveness of distracters is not merely dependent upon common features of the target and distracters, but can be affected by word frequency (a dimension not related to the task demands).     

Visual search in a multi-element asynchronous dynamic (MAD) world

In visual search tasks participants search for a target among distractors in strictly controlled displays. We show that visual search principles observed in these tasks do not necessarily apply in more ecologically valid search conditions, using dynamic and complex displays. A multi-element asynchronous dynamic (MAD) visual search was developed in which the stimuli could either be moving, stationary, and/or changing in luminance. The set sizes were high and participants did not know the specific target template. Experiments 1 through 4 showed that, contrary to previous studies, search for moving items was less efficient than search for static items and targets were missed a high percentage of the time. However, error rates were reduced when participants knew the exact target template (Experiment 5) and the difference in search efficiency for moving and stationary targets disappeared when lower set sizes were used (Experiment 6). In all experiments there was no benefit to finding targets defined by a luminance change. The data show that visual search principles previously shown in the literature do not apply to these more complex and "realistically" driven displays.     

The effects of the binocular disparity differences between targets and maskers on visual search

A visual search for targets is facilitated when the target objects are on a different depth plane than other masking objects cluttering the scene. The ability of observers to determine whether one of four letters presented stereoscopically at four symmetrically located positions on the fixation plane differed from the other three was assessed when the target letters were masked by other randomly positioned and oriented letters appearing on the same depth plane as the target letters, or in front, or behind it. Three additional control maskers, derived from the letter maskers, were also presented on the same three depth planes: (1) random-phase maskers (same spectral amplitude composition as the letter masker but with the phase spectrum randomized); (2) random-pixel maskers (the locations of the letter maskers’ pixel amplitudes were randomized); (3) letter-fragment maskers (the same letters as in the letter masker but broken up into fragments). Performance improved with target duration when the target-letter plane was in front of the letter-masker plane, but not when the target letters were on the same plane as the masker, or behind it. A comparison of the results for the four different kinds of maskers indicated that maskers consisting of recognizable objects (letters or letter fragments) interfere more with search and comparison judgments than do visual noise maskers having the same spatial frequency profile and contrast. In addition, performance was poorer for letter maskers than for letter-masker fragments, suggesting that the letter maskers interfered more with performance than the letter-fragment maskers because of the lexical activity they elicit.     

The consequence of a limited-capacity short-term memory on repeated visual search

ABSTRACT

When participants search the same letter display repeatedly for different targets we might expect performance to improve on each subsequent search as they memorize characteristics of the display. However, here we find that search performance improved from a first search to a second search but not for a third search of the same display. This is predicted by a simple model that supports search with only a limited capacity short-term memory for items in the display. To support this model we show that a short-term memory recency effect is present in both the second and the third search. The magnitude of these effects is the same in both searches and as a result there is no additional benefit from the second to the third search.     

Congruency effects in the letter search task: Semantic activation in the absence of priming

Semantic priming is typically eliminated when participants perform a letter search on the prime, suggesting that semantic activation is conditional upon one's attentional goals. However, in such studies, semantic activation (or the lack thereof) is not measured during the letter search task itself but, instead, is inferred on the basis of the responses given to a later target. In the present study, direct online evidence for semanticactivation was tested using words whose meaning should bias either a positive or a negative response (e.g.,present vs. absent). In Experiment 1, a semantic congruency effect was obtained, with faster responses when the word meaning matched the required response. Experiment 2 replicated the congruency effect while, simultaneously, showing the elimination of semantic priming. It is concluded that letter search does not affect the initiation of semantic activation. Possible accounts for the elimination of priming following letter search include activation-based suppression and transfer-inappropriate processing.     

Reset a task set after five minutes of mindfulness practice

This study aimed to evaluate the impact of a brief mindfulness practice on reducing the carryover effect caused by a previous task set and to determine the mechanism for its effectiveness. Experiment 1 showed that a memorized color interfered with subsequent visual search as a singleton distractor only when color was a defining feature for the search target. In Experiment 2, three interventions (scene-viewing, distraction, and mindfulness practice) were implemented across three groups for five minutes between two blocks; color was relevant to search in the first block and irrelevant in the second. Only the mindfulness group showed a non-significant carryover effect. Experiment 3 demonstrated that the scene-viewing participants continued adopting a suppressive mode of attentional control on a previously distracting color during letter judgment. In contrast, mindfulness practice could reset a task set. Mindfulness practice could enhance concentration in the present moment via reconfiguring the mode of attentional control.     

Effects of letter-names on visual search

When a subject searches through a list of letters of mixed case in a letter cancellation task search is slowed if the background items include the target letter in the other case. This effect is largest when target and confusing background have visual analogue similarity (e.g., cC) but still obtains when no special visual similarity exists (e.g., aA). In searches for two targets, one of either case, search is facilitated when the targets are both cases of the same letter.     

A response binding effect in visual short-term memory

The current study aimed to investigate the effect of action on the preservation of stored feature bindings. Prior research suggests that stimuli presented after a memory array can disrupt the feature bindings of memory array items. Here, we conducted three experiments to examine whether response to targets disrupts feature bindings. Two of four letters (A, B, C, D) were presented in a memory array, and were followed by a second array containing a single target letter. After either identifying or localizing the target letter, participants were required to report the identity or location of the memory array items. There was a deficit in memory performance involving spatial repetition when participants were required to localize targets, and involving identity repetition when participants were required to identify targets. We conclude that response codes are fundamentally linked to stimulus representations, and can affect retrieval from visual working memory.     

Salience drives overt selection of two equally relevant visual targets

In the present study, we investigated whether salience determines the sequence of selection when participants search for two equally relevant visual targets. To do this, attentional selection was tracked overtly as observers inspected two items of differing physical salience: one a highly salient color singleton, and the other a less salient shape singleton. Participants were instructed to make natural eye movements in order to determine whether two line segments contained within the two singletons were oriented in the same or in different directions. Because both singleton items were task-relevant, participants had no reason to inspect one item before the other. As expected, observers fixated both targets on the majority of trials. Critically, saccades to the color singleton preceded saccades to the less salient shape singleton on the majority of trials. This demonstrates that the order of attentional object selection is largely determined by stimulus salience when task relevance is equated.     

Processing of distractors inside and outside the attentional focus in a priming procedure

A priming procedure was used to study the processing of distractors located either inside (between the location of two targets) or outside (peripherally to the locations of the targets) the focus of attention. The stimuli were five-letter arrays, and participants had to decide whether two marked target letters were the same or different. In Experiments 1 and 2, positive priming was obtained both when targets and in-distractors in primes repeated as targets in probes; negative priming was found when out-distractor primes repeated as targets in probes. In Experiment 3, we also manipulated the match in letter case from primes to probes. In-distractors produced reliable positive priming, irrespective of whether the letters matched in case. In contrast out-distractors produced negative priming but only when the letters had the same case in primes and probes. These results are attributed to a spatial attention process operating (in this case) on low-level visual features, and an object-based selection process that enables more abstract information to be processed for selected stimuli.     

Lack of free choice reveals the cost of multiple-target search within and across feature dimensions

Having to look for multiple targets typically results in switch costs. However, using a gaze-contingent eyetracking paradigm with multiple color-defined targets, we have recently shown that the emergence of switch costs depends on whether observers can choose a target or a target is being imposed upon them. Here, using a similar paradigm, we tested whether these findings generalize to the situation in which targets are specified across different feature dimensions. We instructed participants to simultaneously search for, and then fixate, either of two possible targets presented among distractors. The targets were defined as either two colors, two shapes, or one color and one shape. In one condition, only one of the two targets was available in each display, so that the choice was imposed. In the other condition, both targets would be present in each display, which gave observers free choice over what to search for. Consistent with our earlier findings, switch costs emerged when targets were imposed, whereas no switch costs emerged when target selection was free, irrespective of the dimension in which the targets were defined. The results are consistent with the operation of different modes of control in multiple-target search, with switch costs emerging whenever reactive control is required and being reduced or absent when displays allow for proactive control.     

Searching in the dark: Cognitive relevance drives attention in real-world scenes

We investigated whether the deployment of attention in scenes is better explained by visual salience or by cognitive relevance. In two experiments, participants searched for target objects in scene photographs. The objects appeared in semantically appropriate locations but were not visually salient within their scenes. Search was fast and efficient, with participants much more likely to look to the targets than to the salient regions. This difference was apparent from the first fixation and held regardless of whether participants were familiar with the visual form of the search targets. In the majority of trials, salient regions were not fixated. The critical effects were observed for all 24 participants across the two experiments. We outline a cognitive relevance framework to account for the control of attention and fixation in scenes.     

Attention capture while switching search strategies: Evidence for a breakdown in top-down attentional control

Whereas capture experiments typically repeat a single task many times, real world cognition is characterized by frequent switching. Lien, Ruthruff, and Johnston (2010) reported that the attentional control system can rapidly and fully switch between different search settings (e.g., red to green), with no carryover and no intertrial priming. The present study examined whether this impressive flexibility is possible even when the switch is not between different features along the same dimension, but between mutually incompatible search modes. On each trial, participants were prompted to find and identify the letter that was in a specific colour (feature search mode) or was uniquely coloured (singleton search mode). Within each block, search mode was either pure or mixed; the mixed blocks contained a fixed AABB search sequence (singleton-singleton-feature-feature) in Experiment 1 and a random sequence in Experiment 2. The target display was preceded by a noninformative cue display containing a nontarget colour singleton. In pure feature search blocks, these irrelevant singleton cues were generally unable to capture attention, replicating previous findings of “contingent capture”. In mixed blocks, however, irrelevant colour singletons captured attention on feature search trials. This breakdown indicates a limitation in the sharpness of attentional control following mode switches, which might be common in the real world.     

Inhibition of Return Is a Foraging Facilitator in Visual Search

Using overt orienting, participants searched a complex visual scene for a camouflaged target (Waldo from the "Where's Waldo?™" books). After several saccades, we presented an uncamouflaged probe (black disk) while removing or maintaining the scene, and participants were required to locate this probe by foveating it. Inhibition of return was observed as a relative increase in the time required to locate these probes when they were in the general region of a previous fixation, but only when the search array remained present. Perhaps also reflecting inhibition of return, preprobe saccades showed a strong directional bias away from a previously fixated region. Together with recent studies that replicate the finding of inhibition at distractor locations following serial but not parallel visual search—so long as the search array remains visible—these data strongly support the proposal that inhibition of return functions to facilitate visual search by inhibiting orienting to previously examined locations.     

Category effects in visual search: A failure to replicate the “oh-zero” phenomenon

In the experiment of Jonides and Gleitman (1972), subjects searched displays of digits or letters for single, specified digit or letter targets. The slope of the function relating reaction time to display size was positive (mean=25 msec/item) if target and nontargets belonged to the same alphanumeric category (within-category search), but zero if target and nontargets belonged to different categories (between-category search). This held even for the target O, whose categorical relationship to nontargets was determined entirely by the name it was given. In the present paper, two attempted replications are reported, one as close as practically possible. For the unambiguous targets A, Z, 2, and 4, slopes were greater in within-category search than in between-category search, but positive and very variable in both cases. For the ambiguous target O, slopes were identical in within-category and between-category search, and again positive. The results suggest that with single, specified targets, differences between within-category and between-category search may be due entirely to variation in the average physical resemblance between target and nontargets. In line with previous findings, they show that one cannot characterize within-category search as generally “serial” and between-category search as generally “parallel.”     

Spatial selectivity in visual search

To what extent does successful search for a target letter in a visual display depend on the allocation of attention to the target’s spatial position? To investigate this question, we required subjects to discriminate the orientation of a briefly flashed U-shaped form while searching for a target letter. Performance operating characteristics (POCs) were derived by varying the relative amounts of attention subjects were to devote to each task. Extensive tradeoffs in performance were observed when the orientation form and target letter occurred in nonadjacent display positions. In contrast, the tradeoff was much more restricted when the two targets occurred in adjacent positions. These results suggest that the interference between simultaneous visual discriminations depends critically on their separation in visual space. Both visual search and form discrimination require a common limited-capacity visual resource.